1. Field of the Invention
This invention relates to a thin-film semiconductor element such as a thin film transistor, a thin film diode, etc. for use in a switching element, an integrated circuit, a liquid crystal display element, and so on.
2. Description of Prior Art
A thin film semiconductor element such as a thin film transistor, a thin film diode or the like has been researched and developed as an element which can be formed on an insulating substrate, for example, in a driving circuit for a liquid crystal display device or an amplifier for an image sensor. Likewise, in a monolithic technique in which circuit elements are formed on a semiconductor element or semiconductor substrate, the thin film transistor has been also tentatively used to improve integration of the circuit elements and obtain a cubic structure of the circuit. As a thin-film semiconductor material for the above technique has been adopted polycrystalline material such as Polysilicon, amorphous material such as amorphous silicon, or semi-amorphous semiconductor such as semi-amorphous silicon which is intermediate in material property between the polycrystalline material and the amorphous material and has both of polycrystalline and amorphous properties.
However, the carrier mobility of the thin film semiconductor as described above is remarkably small, for example, one-several to one-several tens of that of monocrystal material, and thus the operating speed of a semiconductor element using the semiconductor materials as described above is also remarkably low. For example, the amorphous silicon has an electron mobility below 1 cm.sup.2 /Vs, and a general polysilicon has an electron mobility of 10 to 30 cm.sup.2 /Vs. Even if a special method such as a laser annealing method is adopted, the electron mobility is limited to 200 cm.sup.2 /Vs at the maximum, and this value is remarkably small in comparison with 1350 cm.sup.2 /Vs, the electron mobility of monocrystal silicon. Therefore, the thin film semiconductor element has been mainly used in a relatively-low frequency field or as an auxiliary element for the monocrystal semiconductor such as a load resistance element of a static RAM.
It would be considered as a cause of the low carrier mobility of the thin film semiconductor that a carrier scattering is liable to occur in the amorphous material because the amorphous material has a short crystal periodicity, and the mean free path of carriers becomes shorter. On the other hand, it would be considered for the polycrystalline material that foreign elements are concentrated at grain boundaries and a barrier is liable to occur at the grain boundaries, so that the carriers are randomly scattered at the grain boundaries. On the basis of the above consideration has been made an attempt that each crystal is designed to be larger in size to reduce the number of grain boundaries per unit length, whereby the mobility is increased. The semi-amorphous material mainly comprises a portion having long periodicity as a whole, like the poly-crystalline material, and has no distinct grain boundaries, so that the carrier scattering at the grain boundaries is depressed, and a relatively-high mobility is obtained. However, it is difficult to obtain semi-amorphous material having large grain diameter (an area in which an orderly state is kept over a long distance). In addition, polysilicon having large grain diameter is easily obtained, but characteristics of an element is greatly scattered because the size of the element and the grain diameter are in the same dimension. Such an element can not be practically used.